Toll-like receptor 2 mediates high-fat diet-induced impairment of vasodilator actions of insulin

Abstract

Jang H, Kim H, Hwang DH, Quon MJ, Kim J. Toll-like receptor 2 mediates high-fat diet-induced impairment of vasodilator actions of insulin. Am J Physiol Endocrinol Metab 304: E1077-E1088, 2013. First published March 26, 2013; doi:10.1152/ajpendo.00578.2012Obesity is characterized by a chronic proinflammatory state that leads to endothelial dysfunction. Saturated fatty acids (SFA) stimulate Toll-like receptors (TLR) that promote metabolic insulin resistance. However, it is not known whether TLR2 mediates impairment of vascular actions of insulin in response to high-fat diet (HFD) to cause endothelial dysfunction. siRNA knockdown of TLR2 in primary endothelial cells opposed palmitate-stimulated expression of proinflammatory cytokines and splicing of X box protein 1 (XBP-1). Inhibition of unfolding protein response (UPR) reduced SFA-stimulated expression of TNF␣. Thus, SFA stimulates UPR and proinflammatory response through activation of TLR2 in endothelial cells. Knockdown of TLR2 also opposed impairment of insulin-stimulated phosphorylation of eNOS and subsequent production of NO. Importantly, insulin-stimulated vasorelaxation of mesenteric arteries from TLR2 knockout mice was preserved even on HFD (in contrast with results from arteries examined in wild-type mice on HFD). We conclude that TLR2 in vascular endothelium mediates HFD-stimulated proinflammatory responses and UPR that accompany impairment of vasodilator actions of insulin, leading to endothelial dysfunction. These results are relevant to understanding the pathophysiology of the cardiovascular complications of diabetes and obesity. endothelial function; vascular insulin resistance; Toll-like receptor 2; unfolding protein response; inflammation; endothelial nitric oxide synthase INSULIN SIGNALING PATHWAYS involving insulin receptor/insulin receptor substrates (IRS)/phosphatidylinositol (PI) 3-kinase/ protein kinase B (Akt) contribute to glucose uptake in skeletal muscle and adipose tissue. A similar insulin signaling pathway in vascular endothelium leads to activation of endothelial nitric oxide synthase (eNOS) and production of nitric oxide (NO), which contribute to vasodilation and capillary recruitment (18). Vasodilator actions of insulin play important roles in antiatherogenesis and capillary recruitment that regulate hemodynamics as well as nutrient metabolism (23, 51). Impairment of vascular actions of insulin leads to a reduction of eNOS activity and endothelial dysfunction that contributes to both metabolic dysfunction and cardiovascular complications. This is evident from the metabolic and cardiovascular phenotype of eNOS knockout mice (9,18,43). High-fat diet (HFD) pro-

Toll-like receptor 4-induced endoplasmic reticulum stress contributes to impairment of vasodilator action of insulin

“…We (herein, Figs. 1 and 2) and others (5,22,24) of TLR2 or TLR4 prevents the SFA-induced proinflammatory response in vascular endothelial cells. Interestingly, there was no additive effect when either TLR2 or TLR4 was deleted.…”

“…4D). To examine the role of TLR4 in impairment of insulin-stimulated NO production, bovine aortic endothelial cells (BAEC) were transfected with vector only or dominant negative DNA construct of TLR4 (TLR4-DN) and then examined for insulin-stimulated NO production using the fluorescent NO-specific dye DAF2-DA, as we previously reported (22,26). Insulin-stimulated NO production was inhibited by treatment with palmitate.…”

“…SFAs, including palmitate, stimulate TLRs, which contributes to endothelial dysfunction and insulin resistance (22,24,34,53,60). Both TLR2 and TLR4 are known to be stimulated by SFAs (33, 34).…”

Regulation of obesity and insulin resistance by nitric oxide

“…Elevated free fatty acids lower NO bioavailability in cultured cells [180], isolated arteries [181], animal models [182] and humans [183, 184] and this is likely due to decreased phosphoinositide 3-kinase (PI3K)-Akt-mediated activation of NOS3 [185]. Insulin resistance due to FFAs may be engendered by activation of Toll-like receptor 4 (TLR4) and NF-κB [170, 180] or Toll-like receptor 2 (TLR2) [186]. Other nutrient conditions inherent to diabetes may also be responsible for loss of S1177-NOS3 phosphorylation.…”

The anti-obesogenic effects of nitric oxide.

“…In particular, eNOS phosphorylation at serine 1177 (S1177; S1176 in mice), which is critical for increasing NO output from the enzyme 203 , is diminished in mice by nutrient excess [204][205][206][207] or high fat feeding 155,160,208,209 ; studies in obese rats [210][211][212] 221,222 . Insulin resistance due to FFAs may be engendered by activation of Toll-like receptor 4 (TLR4) and NF-κB 208,218 or Toll-like receptor 2 (TLR2) 223 .…”